1. Field of the Invention
The present invention relates to a process for preparing blast furnace cokes wherein a large amount of lower-quality coals can be used.
2. Description of the Prior Art
Recently there have been many technical developments directed towards the use low-grade coals in the formulation of blast furnace cokes, i.e., coals that previously were considered not at all suitable for such use in blast furnaces. Among them, one development was to attempt to include low-grade coals in coke formulations created by briquetting a coal blended with a binder, and then blending the resulting briquettes with a coal ready for coke oven charging (simply called "after charging coal") to prepare a blended coal ready for coke oven charging (simply called "after blended charging coal"). In this technique, however, the blending ratio of the low-grade coal was restricted since the strength of the coke was decreased as the blending ratio of the low-grade coal was increased.
This can be explained as follows: When a charging coal of conventional grade is blended with briquettes, the bulk density of the blended charging coal is increased, and the strength of the coke which is obtained by carbonizing the blended charging coal is also increased as the blending ratio of the briquettes is increased, i.e., until the blending ratio of the briquettes is greater than a certain amount, in general about 50%. At this amount the spaces formed between the briquettes are not filled up sufficiently with the charging coal, and the bulk density is rather decreased, and thus the strength of the cokes are reduced.
Regarding the above situation, it must also be realized that when blending briquettes with a charging coal, the charging coal particles and the briquettes (the particles and briquettes being of course different in shape and weight) act differently when conveyed from one place to another, when placed in storage, or when taken out of storage etc., and therefore a partial segregation of briquettes in the blended charging coal occurs. For example, when a charging coal of conventional grade is blended with briquettes having an external size of 30-40mm, a weight of 30-40g and a specific gravity of about 1.2, the heavy briquettes, when conveyed, are discharged from the end of a conveyor at a greater speed than that of the charging coal particles, and when the charging coal is placed in the storage, briquettes will immediately and rapidly fall or slip down the dumped surface. Moreover, when the blended charging coal is taken out of a coal bunker for placement in a charging car, due to segregation and the slower moving speed of the briquettes in the hopper, the amount of briquettes in the bunker discharge flow will be increased at the end of the removal period. This latter observation was made during our research studies wherein a blended charging coal, obtained by uniformly blending 30% of briquettes with a charging coal, was charged into a hopper having a capacity of 70 tons, and then 2 ton portions of the blended charging coal were taken out from a discharge means 30 separate times and the amount of briquettes the in the discharge flow was determined each time. It was was determined that the amount of briquettes in the discharge flow varied in the range of 15-50% during the course of each separate coal withdrawal.
Therefore, disregarding the variation of briquettes in the subsequent charging step into the carbonization chamber of a coke oven, when the average amount of briquettes in the blended charging coal is greater than about 30%, and especially when the amount of briquettes exceeds 45-50%, the bulk density and the strength of the cokes produced are, as mentioned above, significantly decreased. For this reason, when a blending process briquettes is practiced on an industrial scale, the average amount of briquettes in the blended charging coal is generally controlled within the limit of about 30%.
Because the strength of blast furnace coke is improved by suitably selecting the amount of briquettes in the blended charging coal as described above, even when a low-grade coal, (which has hitherto been considered not suitable for the preparation of blast furnace coke) is used in the coke, an equal effectiveness of the resulting coke can be maintained by blending in briquettes. That is, low-grade coal can in general be used up to about 10-20% of the blast furnace coke by employing a process for blending in of briquettes.
As a result of our various experiments for the purpose of study on the limit content of low-grade coal in a charging coal, it was found that when blending low-grade coal in the charging coal up to an amount of about 20%, the coke obtained kept its strength when about 30% of the charging coal was briquetted. On the other hand, when the blending ratio of the low-grade coal exceeded about 20%, the coke obtained was decreased in strength at elevated temperatures even if it included blended-in briquettes. Thus, it was impossible to prepare cokes which would endure for use in modern large blast furnaces with respect to its strength at elevated temperatures when low-grade coals were blended in an amount of more than about 20%, even when a portion of this charging coal was in the form of briquettes.
Generally, low-grade coal, such as non-and/or poorly coking coal, has a high chemical activity even after being coked since its constituent carbons are arranged in a three-dimensional structure. Therefore, upon increasing the blending ratio of low-grade coal, in the charging coal the resulting coke has a high gasification speed in the blast furnace, and the embrittlement of the coke itself and its size reduction due to the load and impact within the blast furnace are promoted, which becomes a obstructing factor in operating the blast furnace.
Having thus described, by using a charging coal blended with the low-grade coal where a portion, i.e. about 30%, of the charging coal has previously been briquetted, the dense coke structure of small homogeneous pore and high specific gravity can be obtained to inhibit the chemical reactivity of the coke and to improve the embrittlement in high temperature area within the blast furnace. However, if the blending ratio of the low-grade coal becomes more than about 20%, the activation inhibitory effect by the briquettes is not exerted and the hot properties of the coke become rapidly worse.
As a result of our earnest study on the above-mentioned problems, it was found that the degradation of the hot properties of coke could be inhibited markedly even when the ratio of low-grade coal in the blended charging coal is raised to 30-40%, when the low-grade coal alone or a mixture which is obtained by blending about 40% or more, based on a charging coal, of the low-grade coal, is briquetted in its entirety, that is, the whole amount of the low-grade coal is made to exist only in the briquettes. This is opposed to the situation wherein the low-grade coal is mixed uniformly with the charging coal and then a portion of the mixture is briquetted. The advantage is due to the fact that the activation inhibitory effect by briquetting, namely the density effect of the coke structure by pressure, as well as the modification effect by the binder, have an influence on the whole particles of the low-grade coal.
However, the amount of the briquettes in a blended charging coal is limited to about 30% (as described above), so that even if the briquettes are prepared from low-grade coal alone as suggested above, it would be impossible to make the amount of the low-grade coal in the blended charging coal more than about 30%. Thus, in practice the low-grade coal is used in an amount at most of 15-20%, depending on the properties of the low-grade coal, and an increase in the amount used of the low-grade coal cannot be expected. Consequently, for the purpose of preparing cokes from such blended charging coal on an industrial scale, it is required that the variation of the briquette content resulting from the handling steps of the blended charging coal such as transportation, storage and draw out is made small and at the same time the reduction of the bulk density be inhibited.
If an improved technique were established, for effectively increasing the amount of low-grade coal in briquette-blended high quality cokes, the iron and steel industry would be very much advanced, in the sense of saving natural resources and in a financial sense. The development of such a technique is very much needed.
Therefore an object of the present invention to provide a process for preparing blast furnace cokes blended with a large amount of low-grade coal.